1. Field of the Invention
The present invention relates to an electronic control unit capable of reducing the stress which is developed at juncture portions between a circuit board secured to a chassis of the electronic control unit and lead portions of a bus bar as the temperature changes.
2. Discussion of the Related Art
In a known electronic control unit of this kind as disclosed in U.S. Pat. No. 6,155,856 (equivalent of Japanese unexamined, published patent application No. 2000-164276), a circuit board is fixed with a predetermined distance secured relative to a board-side attaching surface of a plate portion of a chassis or case. A fixing portion of each connector terminal is inserted into an insertion hole from a reverse surface side of the plate portion to be fixed to the insertion hole, and a connection pin of each connector terminal is joined to the circuit board by soldering. In the electronic control unit, a change in temperature causes a stress to be developed at a juncture portion of the connection pin to the circuit board due to the differences in thermal expansion coefficient among the chassis, the circuit board, the connector terminal and the like. In order to reduce the stress, in the known device, a buffering portion or elastic bendable portion is provided between the fixing portion and the connecting pin to absorb the relative displacement which occurs between the circuit board and the plate portion with a change in temperature.
In general, each power line for an electric motor or the like is large in electric current flowing therethrough and therefore, is constituted by a bus bar which is large in width as well as thickness. Where such a bus bar large in width and thickness is joined by soldering to a circuit board without being changed in dimension and shape, the thermal expansion of the bus bar due to a temperature change would cause the soldered juncture portion to be cracked or broken. To obviate this drawback, it has been a practice to divide a base portion of the bus bar connected to a power line for the electric motor, into two lead portions at branch portions on the base portion and to join these lead portions to the circuit board of the electronic control unit. In this case, because of being large in width and thickness, the bus bar cannot be embeded in the plate portion of the chassis. Thus, the bus bar is usually detachably fixed at a fixing portion of its base portion to a board-side attaching surface of the plate portion of the chassis, wherein it has been desired to reduce stresses which are developed at the juncture portions of the two lead portions to the circuit board with a change in temperature.
However, in accordance with the technology described in the foregoing United States patent, it is unable to separate the juncture portions of the two lead portions to the circuit board and to flexibly connect the juncture portions respectively to the fixing portion. The reasons are that the bus bar is detachably fixed to the board-side attaching surface of the plate portion, that the distance between the bus bar and the circuit board is short, and that those parts which extend between the juncture portions of the lead portions to the circuit board and the branch portions at which the lead portions are branched from the base portion act as a single surface which is strong in rigidity.